Among all the renewable energy sources, the contribution of hydropower to the worldwide electricity generation is the highest. A hydropower system can be used to generate a few kilowatt of electricity to about 18,000 MW. Although there are a number of advantages and benefits of using hydropower systems, various environmental issues are restricting their development. These issues include depletion of nutrients in the water body, obstruction of the fish ladder for salmon, and fish shearing in turbines. In this chapter, a general description of a hydropower system and its various components are discussed.